The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for reducing electromagnetic interference (EMI). Merely by way of example, the invention has been applied to power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
With the development of modern electronic technology, more and more electronic devices often operate in a same working environment. In some circumstances, the electronic devices are usually placed very close to each other, and thus problems of electromagnetic interference (EMI) may become severe. Many countries around the world issue standards for electromagnetic compatibility, and electromagnetic compatibility is often deemed as indicating product quality.
Power conversion systems are widely used for consumer electronics (e.g., portable devices) because of the advantages of the power conversion systems such as small sizes, light weight, and high efficiency. FIG. 1 is a simplified diagram showing a conventional switch-mode conversion system. A switch-mode conversion system 100 includes a system controller 180, a switch 140, resistors 150, 154 and 156, capacitors 152, 158 and 166, a primary winding 160, a secondary winding 162, a diode 164, and an isolated feedback component 168. The system controller 180 includes a comparator 110, a modulation component 120, a logic control component 122, a driving component 130, and a leading-edge-blanking (LEB) component 132. The system controller 180 further includes terminals 172, 174, 176, and 178.
For example, the modulation component 120 receives a feedback signal 116 from the isolated feedback component 168 and generates a modulation signal 118. The logic control component 122 receives the modulation signal 118 and outputs a signal 124, which is received by the driving component 130. The driving component 130 outputs a signal 134 through the terminal 172 (e.g., terminal Gate) to close (e.g., turn on) or open (e.g., turn off) the switch 140. The switch-mode power conversion system 100 dynamically adjusts the duty cycle of the switch 140 according to the output load in order to achieve a stable output voltage 170.
In another example, the comparator 110 receives and compares an over-current threshold signal 112 (e.g., Vth—oc) and a current sensing signal 114 (e.g., VCS), and sends an over-current control signal 116 to the logic control component 122. When the current of the primary winding is greater than a limiting level, the logic control component 122 outputs the signal 124 in order to open (e.g., turns off) the switch 140 and shut down the switch-mode power conversion system 100.
FIG. 2 is a simplified diagram showing the conventional driving component 130 as part of the switch-mode conversion system 100. As shown in FIG. 2, the driving component 130 includes NAND gates 202 and 206, NOT gates 204 and 208, and two transistors 210 and 212. For example, the transistor 210 is a P-channel field effect transistor (FET), and the transistor 212 is an N-channel FET.
If the signal 124 from the logic control component 122 is at a logic low level, the NAND gate 202 outputs a signal 214 at a logic high level, and the NAND gate 206 outputs a signal 216 at a logic low level. The NOT gate 208 receives the signal 216 and outputs a signal 218 at the logic high level. The transistor 212 is turned on in response to the signal 218 being at the logic high level. The transistor 210 is turned off in response to the signal 214 being at the logic high level. The signal 134 at the terminal 172 (e.g., terminal Gate) is at the logic low level so that the switch 140 is open (e.g., being turned off).
On the other hand, if the signal 124 from the logic control component 122 is at the logic high level, the NAND gate 206 outputs the signal 216 at the logic high level and the NAND gate 202 outputs the signal 214 at the logic low level. The NOT gate 208 outputs the signal 218 at the logic low level. The transistor 212 is turned off in response to the signal 218 being at the logic low level. The transistor 210 is turned on in response to the signal 214 being at the logic low level. The signal 134 at the terminal 172 (e.g., terminal Gate) is at the logic high level so that the switch 140 is closed (e.g., being turned on).
Strong EMI may be generated during the processes of turning on and turning off the switch 140 which is one of the main sources of EMI in the system 100. Some measures may be taken to reduce EMI, such as using a complicated noise filter, connecting an absorption apparatus in parallel, adopting a resonance technology, and implementing an advanced protection shield, but all of these measures usually increase system costs and scales. In addition, frequency jittering can be used to reduce EMI. Frequency jittering may improve conductive EMI, but often cannot effectively reduce radiative EMI. Hence it is highly desirable to improve techniques for reducing EMI.